This invention relates to a method according to the preamble of the independent claim for monitoring the solder paste printing process in the setting and soldering of a circuit board.
Most of the electronic components used today are surface mounted devices. The components are placed on the surface of the circuit board at predetermined places with regard to the wiring pattern. Before the installation of the surface mounted devices, a paste-like solder alloy is printed on the surface of the circuit board at the solder pads of the components. After this, the surface mounted devices are assembled on the circuit board. The components stay where they are put, because the connection pins of the components stick to the paste deposits of the solder pads. When the components have been assembled, the circuit boards are arranged to go through a furnace where the paste deposits melt. When the circuit boards cool down, the paste hardens and the surface mounted devices have been fastened on the surface of the circuit board by means of solder joints.
The above described solder paste printing process is very sensitive and requires constant monitoring and control in order to achieve the optimum solder quality. It is also to be noted that the size of the components has continuosly decreased, and thus also the size of the terminal pins and solder pads and the gaps between them have decreased, and consequently the requirements for the solder paste printing process have also increased.
The solder paste printing process has earlier been monitored randomly by examining the paste printed circuit board with a microscope. With the naked eye, without any instrument, it is difficult or nearly impossible to perceive a difference in the coverage area of paste deposites at the solder pads of normal components, even if the difference is considerable. The object of the invention is to provide a new method for monitoring the solder paste printing process in the component setting and soldering of a circuit board.
According to the invention, in the method for monitoring the solder paste printing process in the setting and soldering of a circuit board in order to evaluate the quality of the paste printing process, at least one paste test pattern is applied on the circuit board. The test pattern consists of a number of test elements, shaped like geometrical plane figures arranged on the surface of the circuit board, which test elements are of varying degree of difficulty in view of the printing process.
The inventive idea is to use geometrical test patterns printed with paste, such as rectangular test patterns, as the indicators of the capability of the solder paste printing process. The shape and/or size and/or gaps between the test elements used in the test patterns varies systematically in a manner such that the test patterns have various degrees of difficulty in printing. The test figures can be classified according to the degree of difficulty in printing, and according to the classes of difficulty, the solder paste printing process used can be classified into predetermined classes in inspection, and the solder paste printing process can be given a ratio that describes its capability.
In this connection, the term circuit board is used to refer to circuit boards in general, meaning both the actual circuit board on which the components are set and soldered, and a circuit board billet which can comprise one or more actual circuit boards. The solder paste test pattern can be arranged on the actual circuit board, if it has room for it, or on the circuit board billet outside the actual circuit board(s) arranged on it. The solder paste test pattern can be arranged separately from the solder pads of the actual components, or, if it has been arranged on the actual circuit board, its test elements can include solder pads used for fastening the components. Thus the test pattern can, in some applications, be arranged among the solder pads so that the solder pads are used to constitute at least a part of the test pattern.
An advantage of the method according to the invention is the fact that by means of it the solder paste printing process can be reliably monitored and verified. When the test pattern is used, random checks of the printed solder pads of the surface mounted devices before the soldering process can be eliminated.
Another advantage of the method according to, the invention is the fact that it can be used to determine the capability of the solder paste printing process and the accuracy of the parameters used in the printing process. These factors can have a considerable improving effect on the quality of the soldering process. Consequently, inspection of the circuit boards with surface mounted devices after the process can be given up or at least substantially reduced.
Furthermore, it is an advantage of the invention that the same test pattern can be used in the production of many kinds of circuit boards, when surface mounted devices are used in the process.
Due to the invention, the monitoring process in the manufacture of circuit boards becomes less complicated. For example, one part of the test pattern, such as one test element, can be used after the soldering as a test point which is connected to an electric circuit. On the other hand, non-contacting inspection methods, such as a camera system, can also be used.
In addition, due to the invention it is possible to get comparable monitoring results of the manufacture of circuit boards, regardless of which product is manufactured on the circuit board production line. The method according to the invention also provides a possibility for automatic monitoring of the solder paste printing process. It is to be noted that since the production line is used to manufacture different circuit boards for different products, with different arrangements of solder pads, there has not been a single, comparable monitoring method for these circuit boards available earlier, but the monitoring criteria had to be defined for each product individually.